Apparatus and method for deep transient resistivity measurement

ABSTRACT

A method of processing electromagnetic signal data includes: receiving transient electromagnetic (TEM) signal data from a downhole tool disposed in an earth formation, the downhole tool including at least one conductive component; estimating an initial bucking coefficient based on relative positions of the at least two receivers; combining the TEM signal data using the initial bucking coefficient to estimate an initial formation signal; selecting a plurality of bucking coefficient values based on the initial bucking coefficient and estimating a plurality of formation signals, each formation signal corresponding to one of the plurality of bucking coefficients; and selecting an optimal bucking coefficient from one of the initial bucking coefficient and the plurality of bucking coefficients based on the plurality of formation signals, the optimal bucking coefficient providing suppression of parasitic signals due to the at least one conductive component.

BACKGROUND

Geologic formations below the surface of the earth may containreservoirs of oil and gas, which are retrieved by drilling one or moreboreholes into the subsurface of the earth. The boreholes are also usedto measure various properties of the boreholes and the surroundingsubsurface formations.

Deep transient logging while drilling (LWD), especially “look-ahead”capability, has been shown to have a great potential in formationevaluation and measurement, such as in predicting over-pressed zones,detecting faults in front of a drill bit in horizontal wells andprofiling salt structures. These applications typically require depth ofinvestigation (DOI) ranges of about 5-50 meters. One challenge thatoccurs during deep transient LWD operations is the generation of aparasitic signal due to conductive drill collars or other downholecomponents.

A variety of techniques have been proposed to reduce this signal inacquired data. Exemplary techniques include using ferrite and coppershielding, using reference signal for calibration purposes and usingasymptotic behavior of the conductive collar time response to filter outthe collar signal.

In the case of the target DOI being up to 50 meters, the conductivecollar signal is typically more than two orders of magnitude greaterthan the formation signal even if ferrite and copper shields are used.Then the accuracy of bucking and filtering may not be sufficient tofacilitate measurements.

SUMMARY

A method of processing electromagnetic signal data includes: receivingtransient electromagnetic (TEM) signal data representing electromagnetic(EM) signals detected by at least two receivers in a downhole tooldisposed in an earth formation in response to inducing a pulsed currentin the earth formation, the downhole tool including at least oneconductive component; estimating an initial bucking coefficient based onrelative positions of the at least two receivers; combining the TEMsignal data using the initial bucking coefficient to estimate an initialformation signal; selecting a plurality of bucking coefficient valuesbased on the initial bucking coefficient and estimating a plurality offormation signals, each formation signal corresponding to one of theplurality of bucking coefficients; and selecting an optimal buckingcoefficient from one of the initial bucking coefficient and theplurality of bucking coefficients based on the plurality of formationsignals, the optimal bucking coefficient providing suppression ofparasitic signals due to the at least one conductive component.

An apparatus for processing electromagnetic signal data includes: adownhole tool configured to be disposed in a borehole in an earthformation, the downhole tool including a conductive carrier, atransmitter, a first receiver disposed at a first axial distance L₁ fromthe transmitter, and a second receiver disposed at a second axialdistance L₂ from the transmitter that is less than the first axialdistance; and a processor configured to receive transientelectromagnetic (TEM) signal data representing a first EM signal S₁(t)detected by the first receiver and a second EM signal S₂(t) detected bythe second receiver in response to inducing a pulsed current in theearth formation. The processor is configured to perform: estimating aninitial bucking coefficient based on at least the first axial distanceL₁ and the second axial distance L₂; combining the first EM signal S₁(t)and the second EM signal S₂(t) using the initial bucking coefficient toestimate an initial formation signal; selecting a plurality of buckingcoefficient values based on the initial bucking coefficient andestimating a plurality of formation signals, each formation signalcorresponding to one of the plurality of bucking coefficients; andselecting an optimal bucking coefficient from one of the initial buckingcoefficient and the plurality of bucking coefficients based on theplurality of formation signals, the optimal bucking coefficientproviding suppression of parasitic signals due to the at least oneconductive component.

A non-transitory computer readable medium includes computer-executableinstructions for processing electromagnetic signal data by implementinga method comprising: receiving transient electromagnetic (TEM) signaldata representing electromagnetic (EM) signals detected by at least tworeceivers in a downhole tool disposed in an earth formation in responseto inducing a pulsed current in the earth formation, the downhole toolincluding at least one conductive component; estimating an initialbucking coefficient based on relative positions of the at least tworeceivers; combining the TEM data using the initial bucking coefficientto estimate an initial formation signal; selecting a plurality ofbucking coefficient values based on the initial bucking coefficient andestimating a plurality of formation signals, each formation signalcorresponding to one of the plurality of bucking coefficients; andselecting an optimal bucking coefficient from one of the initial buckingcoefficient and the plurality of bucking coefficients based on theplurality of formation signals, the optimal bucking coefficientproviding suppression of parasitic signals due to the at least oneconductive component.

BRIEF DESCRIPTION OF THE DRAWINGS

The following descriptions should not be considered limiting in any way.With reference to the accompanying drawings, like elements are numberedalike:

FIG. 1 depicts an exemplary embodiment of a drilling, formationevaluation and/or production system;

FIG. 2 depicts an exemplary embodiment of a downhole tool;

FIG. 3 depicts a structure representing an exemplary configuration ofthe downhole tool of FIG. 2 in an earth formation;

FIG. 4 depicts exemplary transient electromagnetic responses obtained inthe presence of a typical conductive pipe;

FIG. 5 is a flow chart providing an exemplary method of processingelectromagnetic signal data and/or measuring formation properties;

FIG. 6 depicts exemplary electromagnetic receiver signals;

FIG. 7 depicts exemplary formation signals derived according to themethod of FIG. 5 for a homogeneous formation;

FIG. 8 depicts exemplary formation signals derived according to themethod of FIG. 5 for a homogeneous formation;

FIG. 9 depicts exemplary formation signals derived according to themethod of FIG. 5 for a homogeneous formation;

FIG. 10 depicts an electromagnetic measurement tool disposed in anexemplary formation; and

FIG. 11 depicts exemplary formation signals derived according to themethod of FIG. 5 for the formation depicted in FIG. 10.

DETAILED DESCRIPTION

Apparatuses and methods are provided for reducing and/or eliminatingparasitic signal data due to downhole components (e.g., conductive drillcollars, borehole strings or tool components) from electromagnetic (EM)measurement data. In one embodiment, the apparatuses and methodsdescribed herein are utilized with transient EM operations, such asultra-deep resistivity measurement while drilling. An exemplary methodis based on acquiring EM signals from at least a first and second EMreceiver that are axially spaced downhole relative to an EM transmitter.In one embodiment, a first EM signal is generated from the firstreceiver and a second EM signal is generated from the second receiverlocated closer to the transmitter. The second receiver is combined witha coefficient to generate a transformed signal, which can be subtractedfrom the first EM signal to generate a corrected EM signal that is free(or at least substantially free) of the parasitic signal.

In one embodiment, the first and second receivers are utilized as abucking system for effective reduction or elimination of effects ofconductive components (e.g., drill pipes) on signals in transient EMmeasurements (e.g., Pulse Induction LWD). One receiver, such as thefirst receiver, serves as the main measurement receiver, while anotherreceiver, such as the second receiver, is used to compensate forundesirable effects, e.g., a drill pipe parasitic signal. A correctablebucking coefficient is derived from the raw transient measurements takenby electromagnetic receivers. Bucking is performed by measuring signalsby each receiver and then combining the measured signals using thecorrectable bucking coefficient to derive a formation signal that issubstantially unaffected by the conductive component.

Referring to FIG. 1, an exemplary embodiment of a well drilling, loggingand/or production system 10 includes a borehole string 12 that is showndisposed in a wellbore or borehole 14 that penetrates at least one earthformation 16 during a drilling or other downhole operation. As describedherein, “borehole” or “wellbore” refers to a single hole that makes upall or part of a drilled well. As described herein, “formations” referto the various features and materials that may be encountered in asubsurface environment and surround the borehole.

A surface structure 18 includes various components such as a wellhead,derrick and/or rotary table or supporting the borehole string, loweringstring sections or other downhole components. In one embodiment, theborehole string 12 is a drillstring including one or more drill pipesections that extend downward into the borehole 14, and is connected toa drilling assembly 20. In one embodiment, system 10 includes any numberof downhole tools 24 for various processes including formation drilling,geosteering, and formation evaluation (FE) for measuring versus depthand/or time one or more physical quantities in or around a borehole. Thetool 24 may be included in or embodied as a bottomhole assembly (BHA)22, drillstring component or other suitable carrier. A “carrier” asdescribed herein means any device, device component, combination ofdevices, media and/or member that may be used to convey, house, supportor otherwise facilitate the use of another device, device component,combination of devices, media and/or member. Exemplary non-limitingcarriers include drill strings of the coiled tubing type, of the jointedpipe type and any combination or portion thereof. Other carrier examplesinclude casing pipes, wirelines, wireline sondes, slickline sondes, dropshots, downhole subs, bottom-hole assemblies, and drill strings.

The tool 24, the BHA 22 or other portions of the borehole string 12includes sensor devices configured to measure various parameters of theformation and/or borehole. In one embodiment, the sensor devices includeone or more transmitters and receivers configured to transmit andreceive electromagnetic signals for measurement of formation propertiessuch as composition, resistivity and permeability. An exemplarymeasurement technique is a transient EM technique.

In one embodiment, the tool 24, BHA 22 and/or sensor devices includeand/or are configured to communicate with a processor to receive,measure and/or estimate directional and other characteristics of thedownhole components, borehole and/or the formation. For example, thetool 24 is equipped with transmission equipment to communicate with aprocessor such as a downhole processor 26 or a surface processing unit28. Such transmission equipment may take any desired form, and differenttransmission media and connections may be used. Examples of connectionsinclude wired, fiber optic, acoustic, wireless connections and mud pulsetelemetry.

The processor may be configured to receive data from the tool 24 and/orprocess the data to generate formation parameter information. In oneembodiment, the surface processing unit 28 is configured as a surfacedrilling control unit which controls various drilling parameters such asrotary speed, weight-on-bit, drilling fluid flow parameters and others.

In one embodiment, the tool 24 is configured as a downhole logging tool.As described herein, “logging” refers to the taking of formationproperty measurements. Examples of logging processes includemeasurement-while-drilling (MWD) and logging-while-drilling (LWD)processes, during which measurements of properties of the formationsand/or the borehole are taken downhole during or shortly after drilling.The data retrieved during these processes may be transmitted to thesurface, and may also be stored with the downhole tool for laterretrieval. Other examples include logging measurements after drilling,wireline logging, and drop shot logging.

FIG. 2 illustrates an embodiment of the downhole tool 24. The downholetool 24 is disposed in a carrier such as a housing 30. The housing isincorporated as or in a downhole component such as a borehole stringsection, a drill pipe or a drill collar. The housing 30 and/or othercomponent are typically made from a conducting material such as steel.The tool 24 includes a resistivity measurement assembly 32 incorporatingat least one electromagnetic (EM) source and multiple EM receivers. AnEM transmitter 34 (e.g., a transmitter antenna or coil) is configured toemit an electric or magnetic field into the formation 16 and induce amagnetic field response that is measured by one or more EM receivers 36and 38 (e.g., receiver coils). An electric source 40, which may bedisposed downhole or at a surface location, is configured to applyelectric current to the transmitter 34.

In one embodiment, the measurement assembly 32 is configured to performan inductive transient EM measurement operation. The source 40 appliestransient pulses of current to the transmitter 34, which induces currentin the formation 16. The current generates a magnetic field that isdetected by the receivers 36 and 38.

The tool 24 utilizes electromagnetic measurements to determine theelectrical conductivity of formations surrounding the borehole. Varioustypes of tools may be employed to measure formations at various “depthsof investigations” or DOI, which correspond to distances from the tooland/or borehole in a direction perpendicular to an axis of the tooland/or borehole (e.g., the Z axis of FIG. 2), referred to herein as“radial distances.” Transient EM methods are particularly useful forultra-deep investigations (e.g., radial distances of 10 s to hundreds ofmeters from the tool and/or borehole). Typically, voltage or currentpulses that are excited in a transmitter initiate the propagation of anelectromagnetic signal in the earth formation. Electric currents diffuseoutwards from the transmitter into the surrounding formation. Atdifferent times, information arrives at the measurement sensor fromdifferent investigation depths. Particularly, at a sufficiently latetime, the transient electromagnetic field is sensitive only to remoteformation zones and does not depend on the resistivity distribution inthe vicinity of the transmitter.

In one embodiment, the transmitter and the receivers are disposedaxially relative to one another. An “axial” location refers to alocation along the Z axis that extends along a length of the tool 24and/or borehole 14. The first receiver 36 is positioned at a selectedaxial distance L1 from the transmitter 34, and the second receiver 38 ispositioned at a shorter axial distance L2 from the transmitter. Forexample, the first and second distances are selected to have a specificratio, e.g., L1 is twice that of L2.

In one embodiment, the receivers 36 and 38 are identical or at leastsubstantially identical, such that they would measure the same signal ifthe receivers are disposed at the same axial and radial location. Forexample, the receivers 36 and 38 each have the same (or at leastsubstantially the same) configuration parameters. Such parametersinclude the number and diameter of coil windings, the coil material, theeffective area, the magnetic field to voltage conversion factor and/orvoltage gain.

FIG. 3 shows an exemplary structure representing a configuration of thetool 24 with the formation 16. The structure includes a first zone 42substantially defined by a metal drill collar, pipe or other conductivecarrier with conductivity σ₁, a transition layer 44 having aconductivity σ₂, and a remote formation layer 46 having a conductivityσ₃ The magnetic permeability of the entire space is μ. As illustrated,the boundary 48 separating the metal carrier from the transition layerand the boundary 50 separating the regions of transition layer andremote formation share a common Z-axis. As measured from the Z-axis, theradius of boundary 48 is labeled as r_(md), and the radius of boundary50 is labeled as r_(tl). An electromagnetic field is excited by thetransmitter current loop 34 of radius, r_(xt), and is measured byreceivers 36 and 38 of radius r_(xr).

FIG. 4 shows exemplary transient responses obtained in the presence of atypical conductive pipe. The conductivity is σ=1.4*10⁶ S/m. Curves 51,52 and 53 indicate responses at radial distances (perpendicular to the Zaxis) of 1, 2, and 4 meters respectively to a remote boundary (e.g.,boundary 204). Response curve 54 represents the response to a remoteboundary at an infinite distance. Response curve 54 is nearlyindistinguishable from and overlaps response curves at a distance of 6,8 and 10 meters. FIG. 4 illustrates the fact that at late timescorresponding to deep investigation, the conductive pipe signaltypically dominates the transient response of the earth's formations byat least an order of magnitude. Even when using other methods to removethe main part of the conductive pipe signal (e.g., modeling results forthe pipe signal in air, lab measurements of the pipe signal, by usingbucking coil), there remains a part of the pipe signal left due toinstability of the pipe signal caused by the drilling environment. Thecauses of the instability can be temperature dependence of electricconductivity of the pipe, changing effective distance between thetransmitter and the receiver due to bending of the drill pipe, changingeffective cross-sectional area of the receiver and transmitter andothers. The instability of pipe signal may produce low frequency noisecomparable or exceeding the formation signal especially at late times.

FIG. 5 illustrates a method 60 for processing electromagnetic (EM)signal data and measuring parameters of an earth formation usingelectromagnetic signal measurements. The method includes processingand/or analyzing received signals to reduce and/or eliminate the signalcorresponding to conductive downhole components such as drill collars ordrill pipes from EM data, such as transient EM (TEM) data. The method 60includes one or more of stages 61-66 described herein. The method may beperformed continuously or intermittently as desired. The method isdescribed herein in conjunction with the tool 24, although the methodmay be performed in conjunction with any number and configuration ofprocessors, sensors and tools. The method may be performed by one ormore processors or other devices capable of receiving and processingmeasurement data. In one embodiment, the method includes the executionof all of stages 61-66 in the order described. However, certain stages61-66 may be omitted, stages may be added, or the order of the stageschanged.

In the first stage 61, the tool 24 is lowered in the borehole. The tool24 may be lowered, for example, during a drilling operation, LWDoperation or via a wireline.

In the second stage 62, current is applied to the transmitter 34 and thereceivers 36 and 38 receive signals from the formation during a selectedtime interval. An electric current is applied to the transmitter 34,which induces a pulsed electric current in the formation. This currentin turn generates an associated second magnetic field that is measuredby the receivers 36 and 38 over one or more measurement time intervals.It is noted that each receiver signal can encompass one or multiplesignals over one or more time intervals. The first receiver 36 (alsoreferred to as receiver R₁) is considered the main measurement receiver,for which a time domain signal S₁(t) is measured over a selected timeinterval. The second receiver 38 (also referred to as receiver R₂) isused to measure a time domain signal S₂(t) over the selected timeinterval, and is provided to compensate for undesirable parasiticsignals.

The measured signals S₁(t) and S₂(t) are then combined using acorrectable bucking coefficient to derive a corrected formation signalthat is at least substantially unaffected by conductive components suchas a drill pipe. In one embodiment, the combination is a linearcombination.

For example, a transformation is applied to the second receiver signalS₂(t) to generate a transformed signal. The transformed signal is thensubtracted from the first receiver signal S₁(t) to generate a correctedsignal that is entirely or at least substantially entirely free of theportion of the first signal due to the conductive drill pipe or otherdownhole component.

In one embodiment, the second receiver signal S₂(t) is transformed bymultiplying the receiver signal S₂(t) by the bucking coefficient. Thebucking coefficient may be a constant based on, e.g., a ratio betweenthe distance L₁ from R₁, to the transmitter (T) and the distance L₂ fromR₂ to the transmitter T. An exemplary ratio is (L₂)³/(L₁)³.

In the third stage 63, an initial bucking coefficient k is calculatedbased on, e.g., a ratio between L₁ and L₂. In one embodiment, theinitial bucking coefficient k is calculated based on the followingequation:

$\begin{matrix}{{k = {\frac{M_{1}}{M_{2}}\frac{L_{2}^{3}}{L_{1}^{3}}}},} & (12)\end{matrix}$where M₁(t) and M₂(t) are the magnetic moments of the first and secondreceivers, respectively.

An initial signal ΔS₀(t) is calculated by combining the two signalsS₁(t) and S₂(t) using the initial bucking coefficient. This signalΔS₀(t) is referred to an initial formation signal, which is an initialestimation of a corrected formation signal for which the influence of adrill string or other conductive components (e.g., a parasitic signal)is reduced or eliminated. In one embodiment, the initial formationsignal ΔS₀(t) is calculated according to the equation:ΔS ₀(t)=S ₁(t)−kS ₂(t)  (13)

In the fourth stage 64, an optimal bucking coefficient, i.e., a valuefor the bucking coefficient that most substantially eliminates theparasitic signal, is calculated by estimating a plurality of signalsΔS(t) using a plurality of bucking coefficients selected based on thevalue of the initial bucking coefficient. The plurality of buckingcoefficients are selected based on the initial coefficient. For example,a number of coefficients can be selected that are within a selectedpercentage of the initial coefficient value.

An exemplary range of bucking coefficients has a minimum coefficientk_(min)=0.75·k and a maximum coefficient k_(max)=1.25·k. The criteriafor selection is not limited to those described herein; the values ofthe coefficients, the range between k_(min) and k_(max), the number ofcoefficients, and the interval separating the coefficients can be basedaround the initial coefficient using empirical data, such as knowledgeof formation lithology and previously collected data.

For example, a scan is performed by calculating a plurality of signalsΔS(t) in the time interval [t_(min), t_(max)] corresponding to a rangeof coefficients from k_(min) to k_(max). For example, each signal ΔS(t)is calculated for a respective bucking coefficient according to equation(13). The time interval is related to the measurement interval, and maybe equal to the measurement interval or be some subset thereof. Forexample, the time interval is selected such that t_(min)≈0.01 ms andt_(max)≈1 ms.

From the plurality of the signals ΔS(t), one of the signals (referred toas the optimal signal ΔS(t)_(opt)) is selected that corresponds to thesignal that has or approaches a zero crossing (i.e., a time point atwhich the signal crosses or approaches a value of zero) at the latesttime. The bucking coefficient corresponding to this optimal signal isreferred to as an optimal bucking coefficient k_(opt), and is thecoefficient that provides maximum suppression of the drill pipeparasitic signal while maximizing information content from theformation.

In one embodiment, the signals S₁(t) and S₂(t) are statisticallyanalyzed, such as via a data or curve fitting technique, prior tocombining the signals. For example, a least squares fit of S₁(t) andS₂(t) signals is performed before combining the signals according to theformula (13) to estimate the initial formation signal and the pluralityof formation signals.

In the fifth stage 65, the optimal coefficient k_(opt) can be used inconjunction with subsequent electromagnetic measurements to calculateformation signals. The optical coefficient k_(opt) may be recalculatedat any later point, e.g., periodically after a certain number ofmeasurements and/or in response to changing downhole conditions.

In the sixth stage 66, properties of the formation, such as electricalconductivity of the formation, are estimated based on formation signalscalculated using the optimal coefficient. For example, inversion ofcalculated formation signals provides parameters of the surroundingformation including resistivity, distance to an interface in theformation (geosteering), and distance or ahead of a drill.

FIGS. 6-11 illustrate examples showing the validity and usefulness ofthe method 60 for correction of bucking coefficient in transient MWDmeasurements. The following examples utilize a three-coil systemincluding a transmitter T and two axially spaced receivers R₁ and R₂.The spacing L₁ between the transmitter T and the first receiver R₁ is 7meters, and the spacing L₂ between the transmitter T and the secondreceiver R₂ is 5 meters. It is assumed that the magnetic moments of thereceivers are equal (M₁=M₂). In the below examples, under each coil is a0.1 meter long ferrite with μ=100 and a 75 cm copper shield around thedrill pipe. Both copper and ferrite components are centered with respectto each coil.

Referring to FIG. 6, in a first example, the formation includeshomogeneous media with a resistivity of 1 ohm-m. A curve 70 shows asimulated transient signal S₁(t) for the first receiver R₁ and a curve72 shows a simulated transient signal S₂ (t) for the second receiver R₂.An initial bucking coefficient is calculated as k=L₂ ³/L₁ ³=0.3645. Aninitial formation signal is calculated according to formula (13).

FIG. 7 shows the initial formation signal ΔS₀(t) as curve 74. Areference signal 76 is shown that corresponds to the modeled formationsignal with no pipe or other conductive component included. The curves74 and 76 substantially overlap within the measurement time interval andthus no further processing is needed to update the bucking coefficient,i.e., the initial bucking coefficient can be selected as the optimalcoefficient. If additional formation signals are calculated based on arange of bucking coefficients around the initial coefficient, an optimalcurve 78 is derived based on the selection criteria described above (thecurve with the latest zero crossing). This optimal curve 78 slightlybetter coincides with the reference curve 76. Thus, in this example, theoptimal bucking coefficient (k=0.3646) only slightly deviates from theinitial bucking coefficient value of 0.3645.

Referring to FIG. 8, the situation is different if the formation becomesmore resistive. In a second example, the formation includes homogeneousmedia with a resistivity of 10 ohm-m. An optimal bucking coefficientk=0.3627 leads to a formation signal 80, which is much closer to the tothe reference curve 76 than the initial formation signal 82 calculatedusing the initial bucking coefficient.

Referring to FIG. 9, correction using the method 60 becomes even morepronounced when the resistivity of the formation increases. FIG. 9illustrates an example in which the formation includes homogeneous mediahaving a resistivity of 100 ohm-m. A curve 84, corresponding to theoptimal coefficient k=0.3623, has an overlap with the reference curve 76up to a time moment of about 4E-04 s, which is one decade wider than therange of overlap between the reference curve 76 and the curve 86, whichcorresponds to the initial bucking coefficient of k=0.3645.

FIGS. 10 and 11 show an example of how the correction performs when aformation includes two layers and has a boundary that is placed ahead ofthe tool 24 (e.g., a drill bit or other component). In this example, theformation includes a boundary 88 between a formation layer surroundingthe tool having a resistivity Ro₁ of 50 ohm-m, and a formation layerahead of the tool having a resistivity Ro₂ of 1 ohm-m. The boundary inthis example is placed at 29 meters from receiver R₁.

Curves 90 and 76, corresponding to the optimal and referencecoefficients respectively, are presented in FIG. 11. The curve 90,corresponding to the optimal coefficient k=0.3624, has an overlap withthe reference curve 76 up to a time moment of about 1.0E-03 s, while theinitial signal (curve 92) derived from the bucking coefficient ofk=0.3645 coincides with the reference only until the time moment of1.0E-04 s. By utilizing calculation of the optimal bucking coefficientas described herein, the formation measurement is accurate by one decadelonger in time than the initial signal, which might be translated intoabout 3 times greater depth of investigation.

The apparatuses and methods described herein provide various advantagesover prior art techniques. The apparatuses and methods allow forremoving the effects of the drill collar without having to know thechanges in the drill collar that occur during downhole operation. Suchchanges include environmental changes (temperature and pressure) as wellas physical changes such as deformation and vibration.

In addition, calculation of an improved adjustable bucking coefficientas described herein permits stronger suppression of undesirable signalsdue to conductive components while improving information content aboutelectrical properties of a formation. The adjustable bucking coefficientpermits an effective extraction of formation signals fromelectromagnetic measurements. Generally, some of the teachings hereinare reduced to an algorithm that is stored on machine-readable media.The algorithm is implemented by a computer and provides operators withdesired output.

The systems described herein may be incorporated in a computer coupledto the tool 24. Exemplary components include, without limitation, atleast one processor, storage, memory, input devices, output devices andthe like. As these components are known to those skilled in the art,these are not depicted in any detail herein. The computer may bedisposed in at least one of a surface processing unit and a downholecomponent.

In support of the teachings herein, various analyses and/or analyticalcomponents may be used, including digital and/or analog systems. Thesystem may have components such as a processor, storage media, memory,input, output, communications link (wired, wireless, pulsed mud, opticalor other), user interfaces, software programs, signal processors(digital or analog) and other such components (such as resistors,capacitors, inductors and others) to provide for operation and analysesof the apparatus and methods disclosed herein in any of several mannerswell-appreciated in the art. It is considered that these teachings maybe, but need not be, implemented in conjunction with a set of computerexecutable instructions stored on a computer readable medium, includingmemory (ROMs, RAMs), optical (CD-ROMs), or magnetic (disks, harddrives), or any other type that when executed causes a computer toimplement the method of the present invention. These instructions mayprovide for equipment operation, control, data collection and analysisand other functions deemed relevant by a system designer, owner, user orother such personnel, in addition to the functions described in thisdisclosure.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications will be appreciated by those skilled in theart to adapt a particular instrument, situation or material to theteachings of the invention without departing from the essential scopethereof. Therefore, it is intended that the invention not be limited tothe particular embodiment disclosed as the best mode contemplated forcarrying out this invention, but that the invention will include allembodiments falling within the scope of the appended claims.

What is claimed is:
 1. A method of processing electromagnetic signaldata, the method comprising: receiving transient electromagnetic (TEM)signal data representing electromagnetic (EM) signals detected by atleast two receivers in a downhole tool disposed in an earth formation inresponse to inducing a pulsed current in the earth formation, thedownhole tool including at least one conductive component; estimating aninitial bucking coefficient based on relative positions of the at leasttwo receivers; combining the TEM signal data using the initial buckingcoefficient to estimate an initial formation signal; selecting aplurality of bucking coefficient values from a range of buckingcoefficient values, the range of bucking coefficient values includingthe initial bucking coefficient and estimating a plurality of formationsignals, each formation signal corresponding to one of the plurality ofbucking coefficients; and selecting an optimal bucking coefficient fromone of the initial bucking coefficient and the plurality of buckingcoefficients based on the plurality of formation signals, the optimalbucking coefficient providing suppression of parasitic signals due tothe at least one conductive component, the optimal bucking coefficientselected by identifying a formation signal from one of the initialformation signal and the plurality of formation signals that shows asmallest effect due to the at least one conductive component, andselecting a bucking coefficient corresponding to the identifiedformation signal as the optimal bucking coefficient.
 2. The method ofclaim 1, wherein selecting the optimal bucking coefficient includes:scanning the plurality of bucking coefficients ranging from a minimumcoefficient k_(min) to a maximum coefficient k_(max) to calculate theplurality of formation signals corresponding to the plurality of buckingcoefficients from k_(min) to k_(max); and selecting, as the optimalbucking coefficient, the bucking coefficient that corresponds to theformation signal having a zero crossing at the latest time.
 3. Themethod of claim 1, wherein the range of bucking coefficient valuesinclude values within a selected percentage of the initial buckingcoefficient.
 4. The method of claim 1, wherein the initial formationsignal is estimated by linearly combining the EM signals using theinitial bucking coefficient, and each of the plurality of formationsignals is estimated by linearly combining the EM signals using one ofthe plurality of bucking coefficients.
 5. The method of claim 4, whereinthe initial bucking coefficient is calculated based on the followingequation: ${k = {\frac{M_{1}}{M_{2}}\frac{L_{2}^{3}}{L_{1}^{3}}}},$wherein M₁ and M₂ are the magnetic moments of the first and secondreceivers, respectively.
 6. The method of claim 5, wherein the magneticmoments M₁ and M₂ are selected to satisfy the following condition:$\frac{M_{1}}{M_{2}} = {\frac{L_{1}^{3}}{L_{2}^{3}}.}$
 7. The method ofclaim 1, wherein the downhole tool includes a conductive carrier, atransmitter, a first receiver disposed at a first axial distance L₁ fromthe transmitter, and a second receiver disposed at a second axialdistance L₂ from the transmitter that is less than the first axialdistance, and the EM signals include a first EM signal S₁(t) detected bythe first receiver and a second EM signal S₂(t) detected by the secondreceiver.
 8. The method of claim 7, wherein the initial formation signaland each of the plurality of formation signals are calculated based onthe following equation:ΔS(t)=S ₁(t)−k·S ₂(t), wherein k is the initial bucking coefficient orone of the plurality of bucking coefficients.
 9. The method of claim 7,further comprising performing a data fit of the first EM signal S₁(t)and the second EM signal S₂(t) prior to estimating the initial formationsignal and the plurality of formation signals.
 10. An apparatus forprocessing electromagnetic signal data, the apparatus comprising: adownhole tool configured to be disposed in a borehole in an earthformation, the downhole tool including a conductive carrier, atransmitter, a first receiver disposed at a first axial distance L₁ fromthe transmitter, and a second receiver disposed at a second axialdistance L₂ from the transmitter that is less than the first axialdistance; and a processor configured to receive transientelectromagnetic (TEM) signal data representing a first EM signal S₁(t)detected by the first receiver and a second EM signal S₂(t) detected bythe second receiver in response to inducing a pulsed current in theearth formation, the processor configured to perform: estimating aninitial bucking coefficient based on at least the first axial distanceL₁ and the second axial distance L₂; combining the first EM signal S₁(t)and the second EM signal S₂(t) using the initial bucking coefficient toestimate an initial formation signal; selecting a plurality of buckingcoefficient values from a range of bucking coefficient values, the rangeof bucking coefficient values including the initial bucking coefficientand estimating a plurality of formation signals, each formation signalcorresponding to one of the plurality of bucking coefficients; andselecting an optimal bucking coefficient from one of the initial buckingcoefficient and the plurality of bucking coefficients based on theplurality of formation signals, the optimal bucking coefficientproviding suppression of parasitic signals due to the at least oneconductive component, the optimal bucking coefficient selected byidentifying a formation signal from one of the initial formation signaland the plurality of formation signals that shows a smallest effect dueto the at least one conductive component, and selecting a buckingcoefficient corresponding to the identified formation signal as theoptimal bucking coefficient.
 11. The apparatus of claim 10, wherein theinitial formation signal and each of the plurality of formation signalsare calculated based on the following equation:ΔS(t)=S ₁(t)−k·S ₂(t), wherein k is the initial bucking coefficient orone of the plurality of bucking coefficients.
 12. The apparatus of claim10, wherein the initial bucking coefficient is calculated based on thefollowing equation:${k = {\frac{M_{1}}{M_{2}}\frac{L_{2}^{3}}{L_{1}^{3}}}},$ wherein M₁ andM₂ are the magnetic moments of the first and second receivers,respectively.
 13. The apparatus of claim 10, wherein selecting theoptimal bucking coefficient includes: scanning the plurality of buckingcoefficients ranging from a minimum coefficient k_(min) to a maximumcoefficient k_(max) to calculate the plurality of formation signalscorresponding to the plurality of bucking coefficients from k_(min) tok_(max); and selecting, as the optimal bucking coefficient, the buckingcoefficient that corresponds to the formation signal having a zerocrossing at the latest time.
 14. The apparatus of claim 10, wherein therange of bucking coefficient values include values within a selectedpercentage of the initial bucking coefficient.
 15. A non-transitorycomputer readable medium comprising computer-executable instructions forprocessing electromagnetic signal data by implementing a methodcomprising: receiving transient electromagnetic (TEM) signal datarepresenting electromagnetic (EM) signals detected by at least tworeceivers in a downhole tool disposed in an earth formation in responseto inducing a pulsed current in the earth formation, the downhole toolincluding at least one conductive component; estimating an initialbucking coefficient based on relative positions of the at least tworeceivers; combining the TEM data using the initial bucking coefficientto estimate an initial formation signal; selecting a plurality ofbucking coefficient values from a range of bucking coefficient values,the range of bucking coefficient values including the initial buckingcoefficient and estimating a plurality of formation signals, eachformation signal corresponding to one of the plurality of buckingcoefficients; and selecting an optimal bucking coefficient from one ofthe initial bucking coefficient and the plurality of buckingcoefficients based on the plurality of formation signals, the optimalbucking coefficient providing suppression of parasitic signals due tothe at least one conductive component, the optimal bucking coefficientselected by identifying a formation signal from one of the initialformation signal and the plurality of formation signals that shows asmallest effect due to the at least one conductive component, andselecting a bucking coefficient corresponding to the identifiedformation signal as the optimal bucking coefficient.
 16. The computerreadable medium of claim 15, wherein the initial formation signal isestimated by linearly combining the EM signals using the initial buckingcoefficient, and each of the plurality of formation signals is estimatedby linearly combining the EM signals using one of the plurality ofbucking coefficients.
 17. The computer readable medium of claim 15,wherein selecting the optimal bucking coefficient includes: scanning theplurality of bucking coefficients ranging from a minimum coefficientk_(min) to a maximum coefficient k_(max) to calculate the plurality offormation signals ΔS(t) corresponding to the plurality of buckingcoefficients from k_(min) to k_(max); and selecting, as the optimalbucking coefficient, the bucking coefficient that corresponds to theformation signal having a zero crossing at the latest time.
 18. Thecomputer readable medium of claim 15, wherein the range of buckingcoefficient values include values within a selected percentage of theinitial bucking coefficient.
 19. The computer readable medium of claim15, wherein the downhole tool includes a conductive carrier, atransmitter, a first receiver disposed at a first axial distance L₁ fromthe transmitter, and a second receiver disposed at a second axialdistance L₂ from the transmitter that is less than the first axialdistance, and the initial formation signal and each of the plurality offormation signals are calculated based on the following equation:ΔS(t)=S ₁(t)−k·S ₂(t), wherein k is the initial bucking coefficient orone of the plurality of bucking coefficients, S₁(t) is a first EM signaldetected by the first receiver and S₂(t) is a second EM signal detectedby the second receiver.
 20. The computer readable medium of claim 19,wherein the initial bucking coefficient is calculated based on thefollowing equation:${k = {\frac{M_{1}}{M_{2}}\frac{L_{2}^{3}}{L_{1}^{3}}}},$ wherein M₁ andM₂ are the magnetic moments of the first and second receivers,respectively.